Printing Custom Envelopes

I have a C4500 series and I am trying to print on No. 6 3/4 envelopes (3 5/8" x 6 1/2"). I am printing from a Mac in pages. I had no problem printing on No. 10 envelopes. I have the page setup to the custom size. When I try to print on the No. 6 3/4 envelopes I get an error message:

Paper Size Error: The specified custom paper size is not supported in the selected tray.